The invention relates to scintillator materials, to a manufacturing process for obtaining them and to the use of said materials, especially in gamma-ray and/or X-ray detectors.
Scintillator materials are widely used in detectors for gamma rays, X-rays, cosmic rays and particles having an energy of the order of 1 keV and also above this value.
A scintillator material is a material that is transparent in the scintillation wavelength range, which responds to incident radiation by emitting a light pulse.
It is possible to manufacture from such materials, which are generally single crystals, detectors in which the light emitted by the crystal that the detector contains is coupled to a light detection means and produces an electrical signal proportional to the number of light pulses received and to their intensity. Such detectors are used in particular in industry to measure thickness and grammage or coating weight, and in the fields of nuclear medicine, physics, chemistry and oil research.
One family of known scintillator crystals that is used is that of cerium-doped lutetium silicates. Cerium-doped Lu2SiO5 is disclosed in U.S. Pat. No. 4,958,080, and the patent U.S. Pat. No. 6,624,420 discloses Ce2x (Lu1-yYy)2(1-x)SiO5. Finally, U.S. Pat. No. 6,437,336 relates to compositions of the Lu2(1-x)M2xSi2O7 type, where M is at least partly cerium. These various scintillator compositions all have in common a high stopping power for high-energy radiation and give rise to intense light emission with very rapid light pulses.
A desirable additional property is to reduce the amount of light emitted after the incident radiation stops (i.e. delayed luminescence or afterglow). Physically, this phenomenon, well known to those skilled in the art, is explained by the presence of electron traps in the crystallographic structure of the material. The phenomenon of scintillation relies on the photoelectric effect, which creates an electron-hole pair in the scintillator material. Upon recombination on an active site (a Ce3+ site in the aforementioned scintillators), the electron emits photons via a process that generally takes place in much less than one microsecond. The aforementioned scintillators, which are particularly rapid, result in a pulse duration that decreases with a first-order exponential constant of around 40 ns. However, the trapped electrons do not generate light, but their detrapping by thermal excitation (including at room temperature) gives rise to photon emission—the afterglow—, which still remains measurable after times of greater than one second.
This phenomenon may be unacceptable in applications in which it is desired to isolate each pulse, using very short windowing. This is particularly the case with CT (computed tomography) applications (scanners) that are well known in the medical or industrial sectors. When the CT system is coupled to a PET (Positron Emission Tomography) scanner, which is becoming the standard in industry, the poorer resolution of the CT affects the performance of the entire system and therefore the capability of the clinician to interpret the result of the complete PET/CT system. Afterglow is known to be completely unacceptable for these applications.
Compositions of the lutetium silicates type, disclosed in U.S. Pat. No. 4,958,080 (of the LSO:Ce type, using the notation of those skilled in the art) and U.S. Pat. No. 6,624,420 (of the LYSO:Ce type) are known to generate a significant afterglow. In contrast, the compositions disclosed in U.S. Pat. No. 6,437,336 (of the LPS:Ce type) have the advantage of a much weaker afterglow. These results are given for example by L. Pidol, A. Kahn-Harari, B. Viana, B. Ferrand, P. Dorenbos, J. de Haas, C. W. E. van Eijk and E. Virey in “Scintillation properties of Lu2Si2O7:Ce3+, a fast and dense scintillator crystal”, Journal of Physics: Condensed Matter, 2003, 15, 2091-2102. The curve shown in FIG. 1 is extracted from this article and represents the amount of light detected in the form of the number of events (or counts) per mg of scintillator material as a function of time, under X-ray excitation for a few hours. The LPS:Ce composition gives a significantly better result in terms of afterglow.
The behavior of LYSO is very similar to that of LSO from this standpoint. The reduction in this afterglow forms the subject of the present application.
The afterglow property may be demonstrated more fundamentally by thermoluminescence (see S. W. S. McKeever “Thermoluminescence of solids”, Cambridge University Press (1985)). This characterization consists in thermally exciting a specimen after irradiation and measuring the light emission. A light peak close to room temperature at 300 K corresponds to an afterglow of greater or lesser magnitude depending on its intensity (detrapping). A peak at a higher temperature corresponds to the existence of traps that are deeper but less susceptible to thermal excitation at room temperature. This is illustrated in FIG. 2, extracted from the aforementioned article by L. Pidol et al., which shows, in another way, the advantage of a composition of the LPS type in terms of afterglow.
However, compositions of the LPS type have the drawback of a lower stopping power than those of the LSO or LYSO type. This situation stems simply from the average atomic number of the compound and from the density of the associated phase.